Adhesive compositions for bonding footwear are known in the art. Such known compositions comprise significant amounts of organic solvents. Due to increasingly stringent regulations from health, safety, and environmental authorities said organic solvents have to be eliminated. Unfortunately, the number of alternative adhesive compositions, which are suitable for bonding highly non-polar footwear components, such as bonding synthetic polymeric shoe soles to polar components such as leather uppers, is limited. In addition, the conventional bonding technology requires a pre-treatment of the surface of the non-polar substrates in order to obtain an adequate bonding with a primer. This is essential when using highly non-polar synthetic footwear components, made from e.g. polyolefins or styrenic block copolymers, and particularly hydrogenated styrenic block copolymers, e.g. KRATON® G-2705 block copolymer compositions.
An improved solvent-free adhesive compositions for bonding a polar leather layer to a non-polar substrate layer was disclosed in co-pending application EP02016728.4, which shows a combination of excellent bonding properties and more reliable applicability and processability and lower operational costs, e.g. by operating at lower processing temperatures or in an one step process, enabling shorter cycle times. The polymers described therein are grafted with maleic anhydride. These compositions have relative high HM (high-molecular) viscosity and need high application temperatures of about 230 to 260° C.
Now a novel anionic block copolymer based on mono alkenyl arene end blocks and controlled distribution mid blocks of mono alkenyl arenes and conjugated dienes has been discovered and is described in copending, commonly assigned U.S. patent application Ser. No. 60/355,210, entitled “NOVEL BLOCK COPOLYMERS AND METHOD FOR MAKING SAME”. Methods for making such polymers are described in detail in the above-mentioned patent application.
There is need to a solvent-free, hot melt adhesive composition suitable for bonding a polar leather layer to a non-polar substrate that possesses lower HM viscosity and needs application temperatures below 230° C. At the same time the required good bonding between the components must be maintained for a sufficient long time, i.e. sufficiently lifetime of the produced shoes, leather suitcases, composed leather sporting articles such as golf bags and horse saddles, fashion articles (belts, handbags, briefcases). An object of the present invention is therefore to provide said solvent-free adhesive composition. Another object of the present invention is to provide a process for the manufacturing of composed products, comprising a polar leather component bonded to a non-polar synthetic or polymeric substrate component, and use of said solvent-free adhesive composition.
These and other objects were obtained by providing a new solvent-free hot melt adhesive composition.